What is the run length of the porch? how many columns originally supported it? when you use the jack poles, what are you going to be using to prop them against (an existing beam, a temp wood beam?) Without knowing what you are trying to temporarily support, we can't be much help, I am afraid. The key is the weight and the condition of what you are going to be propping up against. If the whole roof is rotten, a support won't help.
MA, Historic Preservation, BS in Architecture
Less is More!